It is known that most microorganisms have an outer cellular structure, which shows strong non-conductivity. Therefore, in most cases it is difficult for electrons to be transferred to an electrode from the electron transfer reaction occurring in the membrane of the cell. Thus, a microbial fuel cell using ordinary microorganisms essentially uses an electron transfer mediator in order to transfer electrons from the electron transfer system of the microorganisms to an electrode. However, such a mediator is generally a toxic substance and may cause secondary pollution. The mediator also has disadvantages in that it (i) may adhere to an electrode to decrease overall efficiency when used for a long time; (ii) is an aromatic compound, which are toxic to microorganisms; and (iii) is expensive. As such, there are limitations to their use.
If such a mediator is not used, as disclosed by the present inventors, electrons and protons are generated during metabolism of microorganisms, in which the electrons can be directly transferred to an electrode without using a mediator. Subsequently, electrons are transferred to a cathode compartment and the protons are transferred to the cathode compartment via a cation-exchange membrane so that they can be oxidized by oxygen with the electrons [see Kim, Byunghong et al., Korean Patent No. 224381 entitled “Biofuel Cell Using Metal Salt-Reducing Bacteria,” U.S. Pat. No. 5,976,719, Japanese Patent No. 3022431 and EP Patent Application No. 97306661.6].
The present inventors have also found that microorganisms other than the metal salt-reducing bacteria used in the above-cited patents, which can directly transfer electrons previously generated from oxidation of organic substances to an electrode (anode compartment) as metal salt-reducing bacteria, are abundantly present in natural systems, particularly wastewater treatment systems, etc., and can be naturally densely cultured without a separate isolation and culture process during the operation of a biofuel cell [see Kim Byunghong et al., PCT Patent Application No. PCT/KR00/00288 entitled “A Biofuel Cell Using Wastewater and Active Sludge for Wastewater Treatment”].
However, in microbial fuel cells developed hitherto, the cathode compartment and anode compartment have been separated from each other. The generation and transfer of electrons and protons by means of bio-reaction in an anode compartment and the consumption of electrons and protons by means of the reaction of 4e−+4H++O2→H2O should occur integrally with each other, and a circuit should be formed for operating microbial fuel cells for wastewater (sewage) treatment in a continuous manner. Thus, a cation-exchange membrane has been used to transfer protons from the anode compartment to the cathode compartment. If microorganisms in an anode compartment are sufficiently cultured during the process, protons are transferred via a cation-exchange membrane while the amount of electrons generated from the anode compartment and then fed to the cathode compartment can be controlled. A cation-exchange membrane, however, has been found to have limitations on the transfer of protons (Gil Geun Cheol et al., Operational parameters affecting the performance of a mediator-less microbial fuel cell, Biosensors & Bioelectronics, 2003, 18, 327-334). Furthermore, protons generated during metabolism of microorganisms in an anode compartment are not smoothly transferred across a cation-exchange membrane. This is primarily due to membrane fouling. Therefore, a pH decrease in the anode compartment typically occurs when a buffer solution is not used therein. Also, it is so expensive that it has cost limitations as a material used for wastewater (sewage) treatment.